The present invention is directed to a laser communication system, and more particularly to providing a filter-detector receiver for such a system which permits use of a wide variety of laser operating wavelengths.
In proposed laser communication systems which employ a laser transmitter, the receiver is typically an interference filter means which permits only the carrier wavelength to reach the detector. Such interference filters, which are typically optical coatings or plural optical layers of different refractive index, can only accept an input beam incident at a small angle from the normal with a narrow bandwidth, which limits their practical use in optical communication systems. Various laser communication systems for military and civilian applications have been proposed in which the laser beam is transmitted through space, the atmosphere, or through water. For such systems to be practical a filter-detector receiver is desired which is highly efficient at a narrow bandwidth, and which filter exhibits a large field of view or acceptance angle.
Recently, it has been suggested that a filter-detector receiver be fabricated using an atomic resonance filter. In such a filter, cesium vapor which has absorption lines at 459.3 and 455.5 nanometers is utilized as the filter for a Raman downshifted xenon chloride laser transmitter. The cesium atoms in the ground state absorb the laser output line, and upon relaxation, emit characteristic detectable 894 nanometer, and 853 nanometer output fluorescence or radiation. This cesium filter is one of a very few atomic species with an absorption line that closely matches a practical laser output wavelength.
Recent laser developments have greatly increased the variety of lasers and operating wavelengths by use of tunable molecular lasers, dye laser, and Raman lasers. A receiver which permits use of the wide variety of operating laser wavelengths is sought.